Thermomechanical Processing of IN 601 And SS 304

Abstract

Present study investigates isothermal annealing behaviour of prior-cold-worked Inconel 601 (aka, IN 601) and Stainless-Steel 304 (aka, SS 304) sheets. The study comprehensively covers the annealing response of the materials over wide cold reduction and temperature ranges. Using structural characterization and mechanical testing, the study tracks strain-hardening, strain-aging, recovery, and recrystallization stages of the sheets as a function of degree of cold-reduction and annealing temperature. By tracking the thermomechanical behaviour of the experimental materials, the study searches the possibility of producing fine-elongated grains in recrystallized-form such that the creep-resistance of wrought alloys should improve without compromising on the strength, toughness, and fatigue-resistance. Using X-Ray diffraction analysis, hardness measurements, and tensile tests, the study reveals that, irrespective of the degree of cold-reduction, both IN 601 and SS 304 consistently exhibit strain-aging during low-temperature exposures as the experimental-results confirm that the prior-coldworked sheets display non-monotonic increasing-decreasing trend in hardness and strength with increased annealing temperature. The investigation reveals that the ‘recovery-stage’ is preceded by ‘strain aging-stage’ during which the alloys exhibit superior strength and hardness than the strain-hardened and recovered states. Discontinuous-yielding observed during the loading-unloading type tensile tests suggest that the peak-hardening due to strain aging during low-temperature annealing is likely caused by dislocation-pinning of interstitial carbon. Based on the thermomechanical experimental-results, the current work proposes a thermomechanical processing map for each alloy that integrates ‘strain-hardening’ and ‘strain-aging’ stages with ‘recovery’ and ‘recrystallization’ stages. Additionally, microstructural analysis and SEM-EBSD analysis presented in this work indicate that, by suitably controlling strain-hardening and the recrystallization annealing, a refined microstructure comprising high aspect-ratio grains having high angle grain-boundaries can be obtained in IN 601 sheets that may improve both fatigue and creep properties. However, SS 304 does not form elongated grains during the recrystallization likely due to the low stacking fault energy of the material that promotes twinning and leads to formation of polygonal equiaxed grains.